Terminal electrode compositions for multilayer ceramic capacitors

ABSTRACT

The present invention relates to terminal electrode compositions for multilayer ceramic capacitors. More specifically, it relates to terminal electrode compositions for multilayer ceramic capacitors, which compositions are made of a copper-based powder and an organic binder that can be fired at a low temperature in a nitrogen atmosphere.

FIELD OF THE INVENTION

[0001] The present invention relates to terminal electrode thick filmcompositions for multilayer ceramic capacitors. More specifically, itrelates to terminal electrode compositions for multilayer ceramiccapacitors (MLC), which compositions are made of a metal-based powderdispersed in an inert liquid organic medium, which is fireable at a lowtemperature in a neutral or reducing atmosphere.

BACKGROUND OF THE INVENTION

[0002] Electrically conductive pastes composed of a base metal such ascopper or nickel dispersed with an inorganic binder and an organicbinder dispersed in a solvent are widely used as terminal electrodecompositions for multilayer ceramic capacitors. These conductive pastecompositions are typically fired in a neutral or reducing atmosphere(such as nitrogen) to suppress oxidation of the constituent metal andinternal electrode. Accordingly, of the ingredients contained in thepaste, it is necessary for the organic binder which must be burned offin the firing step to have sufficient thermal decomposability in thistype of atmosphere. Various types of acrylic polymers are well suitedfor this purpose. For instance, JP-A 2001-307549 describes examples inwhich compounds such as methyl methacrylate, ethyl methacrylate andbutyl methacrylate are used.

[0003] However, prior-art electrically conductive pastes have thefollowing drawbacks. The fired film must have a thickness which issufficiently low to satisfy the chip specifications and achieve goodreliability during mounting. For example, at chip dimensions of 1.2 mm(width)×1.2 mm (thickness)×2.0 mm (length), a terminal film thickness ofnot more than 90 microns is appropriate. The method ordinarily used tocontrol the film thickness within this range is a step called“blotting,” in which excess paste is removed after coating. However,this increases costs due to the increased manufacturing tact time itrepresents and the increase in man-hours required to recycle the pastethat is removed.

[0004] Approaches that have been tried in order to eliminate blottinginclude that of optimizing the paste characteristics to enable theapplication of a thinner film. One such method has involved reducing thepaste viscosity so as to lower the amount of paste deposited to theelement assembly of the MLC. However, the low viscosity allows the pasteto sag on the sidewalls, preventing the shape to be maintained.

[0005] Moreover, although film thickness-reducing effects can beexpected with the use of a method for lowering the inorganic solidscontent within the paste, there is a corresponding increase in organicbinder and thinner components, which lowers the viscosity. Increasingthe amount of organic binder to check this decline in viscosity willensure that a suitable viscosity is achieved, but an excessive increasein the amount of organic binder per unit volume harms thermaldecomposability of the organic binder fired in a neutral or reducingatmosphere.

[0006] It is therefore an object of the invention to provide a thickfilm paste composition for terminal electrodes, which composition has areduced solids content yet maintains a suitable paste viscosity and isable to ensure sufficient thermal decomposability of the organic binder.

DETAILED DESCRIPTION OF THE INVENTION

[0007] The conductive thick film paste composition of the inventioncomprises a specified amount of a methyl methacrylate polymer having asuitable molecular weight, the solids within the paste composition canbe lowered to 75 wt % or less (e.g., 70 wt %), making it possible toreduce the film thickness and thus form the desired terminal electrodes.The invention is described more fully below.

[0008] The organic medium is preferably one prepared by the dissolutionof an acrylic polymer within a suitable solvent. The polymer has a highthermal decomposability within a neutral or reducing atmosphere.

[0009] The polymer used in the invention is methyl methacrylate, but themethyl methacrylate may be combined with polymers selected from ethylmethacrylate and butyl methacrylate, and copolymers of acrylatecompounds, and blends of these listed above.

[0010] The ratio of inert medium to solids in the composition may varyconsiderably and depends upon the manner in which the dispersion of thesolids in medium is to be applied. Dispersion contains 45 to 76 wt %solids and 24 to 55 wt % medium.

[0011] Accordingly, the invention provides a terminal electrodecomposition for multilayer capacitors, which composition comprises 30 to71 wt % of a conductive powder selected from copper powder, nickelpowder and copper-nickel alloy powder dispersed in an organic binder insolvent such as CARBITOL® acetate and butyl CARBITOL® acetate (CARBITOL®is a registered trademark of Union Carbide Chemicals & PlasticsTechnology Corporation), wherein the organic binder is composed of oneor more types of methyl methacrylate (MMA) polymer dissolved in anorganic solvent, at least one of the methyl methacrylate polymers havinga number-average molecular weight of at least 100,000 and aweight-average molecular weight of at least 1,000,000, such that themethyl methacrylate polymer accounts for 2.0 to 9.0 wt % of the paste,based on total composition.

[0012] To remove the organic medium, suitable oxygen doping in thenitrogen firing furnace is ordinarily carried out within a thermaldecomposition temperature range of 150 to 450° C. The methylmethacrylate polymer content for carrying out sufficient thermaldecomposition is not more than 9.0 wt %, and preferably not more than7.0 wt %, based on the total paste. At a methyl methacrylate polymercontent of more than 9.0 wt %, the polymer content per unit volumebecomes to high for the amount of oxygen supplied. As a result, thelevel of unburned organic residues rises, which can cause sinteringdefects.

[0013] The organic medium confers a viscosity which allows the paste tobe deposited on a substrate in an appropriate shape, and also maintainsthe strength of the dried coating. The use of a methyl methacrylatepolymer is especially preferable for achieving a sufficient dry coatstrength even when the polymer is used in a small amount. The use ofless than 2.0 wt % results in an excessive decrease in the viscosity,preventing a sufficient dry film strength.

[0014] In the practice of the invention, the base metal particles areselected from a copper powder, a nickel powder or a copper-nickel alloypowder. A copper powder is preferred. Copper powder particles areselected from spherical or of indeterminate shape and have an averageparticle size of 0.5 to 30 μm, and flake-like copper particles having aparticle size of 0.1 to 30 μm, and mixtures thereof. Base metalparticles that are too large compromise the density of the terminalelectrode produced therefrom. On the other hand, if the particle size istoo small, the dispersion properties differ from those of the organicmedium, giving rise to a change in rheology which makes it difficult toachieve an ideal coated shape.

[0015] The content of base metal particles within the paste is 30 to 71wt %. Below this range, a dense sintered film is not obtained; whereasabove this range, the desired paste viscosity is not achieved.

[0016] Illustrative, preferred, non-limiting examples of the glass fritused in the present invention include those composed of Si—B—Ba glass,Si—B—Pb glass and Si—B—Zn glass. The softening point of the glass fritas an inorganic binder is closely associated with the firingtemperature. Too high a softening point inhibits sintering, whereas toolow a softening point promotes sintering. The firing temperature of thepaste according to the invention is about 700 to 950° C. Hence, whenfiring is carried out at about 750° C., for example, to keep thecomposition from undergoing an excessive degree of sintering yet allowit to achieve a suitable degree of density, it is preferable for theglass softening point be set within a range of 500 to 650° C.

[0017] The paste composition has a glass frit content from 5 to 15 wt %,and preferably from 8 to 12 wt %, based on the overall composition. Whentoo little glass frit is added, a fired film having sufficient densityto serve as a barrier to the plating solution cannot be obtained, andadhesion to the capacitor assembly is inadequate. On the other hand, theaddition of too much glass frit causes glass components to rise to thesurface of the fired film, greatly compromising the plating adhesion.The glass frit is preferably a finely divided powder having a particlesize of 0.5 to 20 μm, and especially 1 to 10 μm. Too large a particlesize results in a low density, whereas too small a particle size resultsin dispersion properties that differ from those of the organic binder,altering the rheology and making it difficult to achieve an ideal coatedshape.

[0018] In the practice of the invention, the above-described base metalparticles and glass frit are dispersed in an organic medium to form apaste composition. The composition is coated onto a terminalelectrode-forming site of a multilayer ceramic capacitor. It is thenfired at a temperature of 700 to 950° C. to form terminal electrodes.Nickel or solder plating is then applied as a soldering surface to theterminal electrodes after they have been fired, thereby giving finishedterminal electrodes.

[0019] By including within the paste 2.0 to 9.0 wt % of at least onetype of methyl methacrylate polymer having a weight-average molecularweight of at least 1,000,000, an electrically conductive paste isobtained that has an adequate dry coat strength, is free of pastesagging at end faces even at a low solids content, and has sufficientthermal decomposability of the organic components when fired in aneutral or reducing atmosphere, and is free of adverse effects uponsintering by unburned organic matter.

EXAMPLES

[0020] Examples of the invention and comparative examples are givenbelow. All percentages are based on total composition.

Example 1

[0021] A methyl methacrylate polymer having a weight-average molecularweight of 1,200,000 (22 wt %) was dissolved in butyl CARBITOL® acetate(78 wt %) to form an organic medium. 30 wt % of the above organicmedium, 63 wt % of spherical copper powder having an average particlesize of 3 μm and 7 wt % of Si—B—Ba glass frit were each weighed out inthe indicated amounts and uniformly dispersed by blending in athree-roll mill to form a paste.

[0022] The Si—B—Ba glass frit had the composition by weight:

[0023] 35.0% BaO

[0024] 23.1% B₂O₃

[0025] 13.5% SrO

[0026] 12.5% SiO₂

[0027] 4.5% ZnO

[0028] 3.7% MgO

[0029] 2.4% Al₂O₃

[0030] 2.3% Na₂O

[0031] 1.2% SnO₂

[0032] 1.0% TiO₂

[0033] 0.4% K₂O

[0034] 0.4% LiO

Example 2

[0035] 15 wt % a methyl methacrylate polymer having a weight-averagemolecular weight of 1,200,000 (22 wt %) was dissolved in butyl CARBITOL®acetate (78 wt %), 10.5 wt % methyl methacrylate having a weight-averagemolecular weight of 200,000 dissolved in butyl CARBITOL® acetate formedthe organic vehicle, 67 wt % of spherical copper powder having anaverage particle size of 3 μm and 7.5 wt % of Si—B—Ba glass frit ofExample 1 were each weighed out in the indicated amounts and uniformlydispersed by blending in a three-roll mill to form a paste.

Example 3

[0036] 15 wt % of a methyl methacrylate polymer having a weight-averagemolecular weight of 1,200,000 (22 wt %) was dissolved in butyl CARBITOL®acetate (78 wt %), 10.5 wt % methyl methacrylate-butyl methacrylatecopolymer having a weight-average molecular weight of 150,000 wasdissolved in butyl CARBITOL® acetate (78 wt %) to form an organicvehicle, 67 wt % of spherical copper powder having an average particlesize of 3 μm and 7.5 wt % of Si—B—Ba glass frit of Example 1 were eachweighed out in the indicated amounts and uniformly dispersed by blendingin a three-roll mill to form a paste.

Comparative Example 1

[0037] 30 wt % butyl methacrylate polymer having a weight-averagemolecular weight of 680,000 (22 wt %) was dissolved in butyl CARBITOL®acetate (78 wt %) to form an organic vehicle of the above organicvehicle, 63 wt % of spherical copper powder having an average particlesize of 3 μm and 7 wt % of Si—B—Ba glass frit of Example 1 were eachweighed out n the indicated amounts and uniformly dispersed by blendingin a three-roll mill to form a paste.

Comparative Example 2

[0038] 30 wt % methyl methacrylate polymer having a weight-averagemolecular weight of 200,000 (22 wt %) was dissolved in butyl CARBITOL®acetate (78 wt %) to form an organic vehicle, 63 wt % of sphericalcopper powder having an average particle size of 3 μm and 7 wt % ofSi—B—Ba glass frit of Example 1 were each weighed out in the indicatedamounts and uniformly dispersed by blending in a three-roll mill to forma paste.

Comparative Example 3

[0039] 16.7 wt % methyl methacrylate polymer having a weight-averagemolecular weight of 200,000 (22 wt %) was dissolved in butyl CARBITOL®acetate (78 wt %) to form an organic vehicle, 75 wt % of sphericalcopper powder having an average particle size of 3 μm and 8.3 wt % ofSi—B—Ba glass frit of Example 1 were each weighed out in the indicatedamounts and uniformly dispersed by blending in a three-roll mill to forma paste.

Test Methods

[0040] Evaluation Tests:

[0041] The pastes formulated as described above were coated ontomultilayer ceramic capacitor chips with dimensions of 1.2 mm (width)×1.2mm (thickness)×2.0 mm (length), fired at a temperature of 750° C. in anitrogen atmosphere to prepare test pieces, and the film thickness atthe ends were measured. In addition, the end faces of the coated chipswere examined for sag and rated as acceptable (OK) or unacceptable (NG).The paste compositions and evaluation results are shown below inTable 1. TABLE 1 EX 1 EX 2 EX 3 CE 1 CE 2 CE 3 Organic Resin MMA MMA MMA— — — vehicle Weight-average ×10,000 120 120 120 — — — 1 molecularweight Polymer amount % 22 22 22 0 0 0 Organic Polymer MMA MMA/BMA BMAMMA MMA vehicle Weight-average ×10,000 — 20 15 68 20 20 2 molecularweight Polymer amount % 0 22 22 22 22 22 Paste Organic vehicle 1 % 30 1515 0 0 0 Organic vehicle 2 % 0 10.5 10.5 30 30 16.7 Glass % 7 7.5 7.5 77 8.3 Copper powder % 63 67 67 63 63 75 Solids content % 70 74.5 74.5 7070 83.3 Polymer content % 6.6 5.61 5.61 6.6 6.6 3.674 Results Filmthickness μm 75 89 88 76 73 121 End face sag OK OK OK NG NG OK

[0042] It is apparent from the results in Table 1 that in Examples 1 to3 according to the invention, each of which includes a type and amountof methyl methacrylate which satisfies the conditions set forth in claim1, sagging of the paste was not observed on the end faces (OK), and thefilm thickness was less than 90 microns. By contrast, in each ofComparative Examples 1 to 3, either the film thickness was greater than90 microns or sagging of the paste was observed on the end faces of thecoated chip (NG).

What is claimed is:
 1. An electrically conductive paste fireable in aneutral or reducing atmosphere comprising (a) 30 to 71 wt % conductivepowder being selected from the group of copper powder, nickel powder andcopper-nickel alloy powder and (b) an inorganic binder, both dispersedin an inert organic medium; wherein the organic medium comprises atleast one methyl methacrylate (MMA) polymer dissolved in solvent, saidmethyl methacrylate polymer having a number-average molecular weight ofat least 100,000 and a weight-average molecular weight of at least1,000,000, such that the methyl methacrylate polymer accounts for 2.0 to9.0 wt % of the paste.
 2. The conductive paste of claim 1 wherein theamount of the inorganic binder is in the range from 5 to 15 wt %, andthe conductive powder and inorganic binder combined is in the range from45.0 wt % to 76 wt %.
 3. The conductive paste of any one of claims 1 or2, wherein the organic medium further comprises ethyl methacrylate,butyl methacrylate, copolymers of acrylate compounds, or mixturesthereof.
 4. The conductive paste of any one of claims 1-3, wherein theinorganic binder is selected from Si—B—Ba glass, Si—B—Pb glass, Si—B—Znglass, or mixtures thereof.
 5. The use of the conductive paste of anyone of claims 1-4 as a terminal electrode composition for multilayercapacitors.
 6. A method of forming a terminal electrode comprising: (a)forming the conductive paste of any one of claims 1-4; (b) coating thecomposition of (a) onto a terminal electrode-forming site of amultilayer capacitor; and (c) firing the multilayer capacitor in (b) toform a finished terminal electrode.
 7. A multilayer capacitor utilizingthe conductive paste of any one of claims 1-4.